Fourier telescopy is an imaging technique that uses multiple beams from spatially separated transmitters to illuminate a distant object. A large number of wide aperture telescopes (referred to as “light buckets”) are used to detect the light reflected from the object with sufficient signal-to-noise ratio, but no attempt is made to form an image directly from the collected light. Each beam is shifted relative to the other beams to produce interference between beams at the target. Thus interference patterns are produced on the target. These interference patterns are encoded in the time dependent reflections from the target. The reflected light collected by the light buckets is Fourier transformed into a frequency spectrum as a function of time and this spectrum is correlated with known positions and frequencies of the transmitted beams as a function of time to derive a two dimensional image of the target.
This imaging technique has been studied extensively for use in imaging deep space objects. In prior art system designs, for example, three beams would be transmitted simultaneously in pulses to image a geosynchronous object. It would take many hours to transmit the tens of thousands of pulses needed to construct all of the spatial frequencies needed to form an image of the object. Because the position and orientation of the object would remain essentially constant, this approach seemed feasible. Three illuminating apertures were used in order to eliminate the degrading atmospheric phase aberrations using the well known technique of phase closure, and then the closure phases used to reconstruct the illuminated target image.
Fourier telescopy is an active imaging method which interferes spatially diverse, frequency-encoded laser beams on a distant target, and records a time history of the reflected intensity measured by a single photodetector on a large receiver. Phase closure among the received triplets plays a key role in canceling out random atmospheric phase errors between laser beams. This imaging technique has been studied extensively for use in imaging deep space objects under previous Air Force programs. See for example: R. B. Holmes and T. J. Brinkley, “Reconstruction of images of deep-space objects using Fourier telescopy,” in Digital Image Recovery and Synthesis IV, T. J. Shultz and P. S. Idell, eds., Proc. SPIE 3815, pp 11-22 (1999); M. Thornton, J. Oldenettel, D. Hult, K. Koski, T, Depue, E. L. Cuellar, et. al, “GEO Light Imaging National Testbed (GLINT) Heliostat Design and Testing Status”, in Free-Space Laser Communication and Imaging, D. G. Voelz and J. C. Ricklin, eds., Proc. SPIE, Vol. 4489, pp 48-59 (2002); and E. Louis Cuellar, James Stapp, Justin Cooper, “Laboratory and Field Experimental Demonstration of a Fourier Telescopy Imaging System”, Proc. SPIE 5896, (2005).
In a particular prior art design, for example, three beams would be transmitted simultaneously in pulses to image a geosynchronous object. It would take many hours to transmit the tens of thousands of pulses needed to construct all of the spatial frequencies needed to form an image of the object. Because the position and orientation of the object would remain essentially constant, this approach seemed feasible. Three illuminating apertures were used in order to eliminate the degrading atmospheric phase aberrations using the technique of phase closure5, and then the closure phases used to reconstruct the illuminated target image2. Previous experiments in both the lab and field have verified that this implementation of the Fourier Telescopy technique to imaging geostationary targets is both viable and robust. See for example: J. Mathis, E. L. Cuellar, J. Cooper, A. Morris, P. Fairchild, D. Hult, K. Koski, L. Ramzel, and M. Thornton, “Field Experiment Performance of the Receiver Elements for a Fourier Telescopy Imaging System” Proc. SPIE 5896, (2005).
What is needed is a system to image fast mobbing far away objects such as satellites in low earth orbits.